I was wondering if you could share how you came up with the power circuit for the chess pieces. I’ve tried looking for resources on wireless power transfer, but I have yet to find a good source of info on it.

I found the same problem when starting on this idea, the circuits you can find are usually very inefficient and can only power a LED. Mostly by experimenting with different drivers, coils and drive frequencies – the signal generator’s driven them at everything from 20khz to 300+ at time ;), it’s been through many shapes and styles of coil, at one point I used a spiral coil that could be etched on a double sised PCB but it wasn’t efficient enough at smaller diameters. Smallest coil I’ve used for nixies was just over 15mm. The driver circuit is the easiest part…..